(1) Field of the Invention
The present invention relates to a granular stabilizer for chlorine-containing polymers and a process for the preparation thereof. More particularly, the present invention relates to a granular stabilizer for chlorine-containing polymers, which is excellent in the dispersibility in a resin, the uniformity of the composition, the resistance to dusting or scattering, the flowability and the blocking resistance even though it is formed by granulating a hardly fusible or infusible stabilizer powder with use of a very small amount of a binder or dispersion medium, and a process for the preparation of this granular stabilizer.
(2) Description of the Prior Art
A stabilizer such as tribasic lead sulfate is incorporated into a chlorine-containing polymer such as a vinyl chloride resin so as to improve the thermal stability thereof. In this case, a hardly fusible or infusible stabilizer should be as fine as possible in view of the desire for dispersibility in a resin, however fine powder involves a problem of scattering. Accordingly, the hardly fusible or infusible stabilizer is widely handled in the form of granules.
For example, Japanese Patent Publication No. 3554/66 proposes a process for the preparation of a lubricating stabilizer for a vinyl chloride resin which comprises adding a high-melting-point powdery stabilizer, infusible at the processing temperature for a vinyl chloride resin or a lower temperature, to a melt of a metal soap which is to be incorporated as a lubricating stabilizer at the step of processing a vinyl chloride resin, mixing them homogeneously to form a solid solution, and shaping the solid solution into granules or small masses. According to this process, the problem of scattering is overcome, and this process is prominently advantageous in that a so-called one-package stabilizer having a uniform composition can be provided. In order to granulate the powdery stabilizer by using the metal soap melt as the binder, it is necessary to use the metal soap in such a large amount as at least 1/3 part by weight of the powdery stabilizer. Accordingly, if this granular one-package stabilizer is incorporated in a vinyl chloride resin in an amount sufficient to attain an intended stabilizing effect, the amount of the metal soap, such as lead stearate, becomes excessive and, at the step of processing the vinyl chloride resin, precipitation of the metal soap is often caused. Therefore, according to the granulation process using a binder which is softened at the resin processing temperature, it is difficult to provide a granular one-package stabilizer in which the mixing ratio of a powdery stabilizer is increased.
A granular one-package stabilizer containing a powdery stabilizer at a relatively high mixing ratio has also been known. For example, in Japanese Patent Publication No. 47138/78, we have proposed a composition comprising a lead pigment, such as tribasic lead sulfate, affined with a liquid plasticizer, such as dioctyl phthalate, in a specific amount relative to the critical liquid absorption of the pigment. This composition has a reduced powder-scattering property and a flowability suitable for handling and is advantageous in that the mixing ratio of the powdery additive in the composition can be increased. However, this composition is defective in that the shape and size of particles of the final composition are not uniform, the mechanical strength of the particles is ordinarily low, and blocking is caused more or less.
Furthermore, Japanese Patent Publication No. 36932/79 discloses a granular stabilizer comprising cores containing a lubricant as a continuous phase and a powdery stabilizer as a dispersed phase and shells formed on the surfaces of the cores, which shells are composed of a powdery stabilizer and a plasticizer in an amount smaller than the critical liquid absorption. This granular stabilizer is advantageous over the first-mentioned granular stabilizer in that the powdery stabilizer can be incorporated at a higher mixing ratio. However, this granular stabilizer is still insufficient in that the blocking tendency cannot completely be eliminated and the dispersibility of the powdery stabilizer into resins is low.
Moreover, Japanese Patent Publication No. 21819/72 discloses a process in which several stabilizers, at least one of which is a fusible substance, are sinter-granulated without melting said fusible substance by using a powder blender provided with a rotor rotated at a high speed. If this sintering granulation is carried out, a homogeneous granulation product cannot be obtained unless the fusible substance is used in a considerable amount.